Speed of light
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The speed of light in a vacuum (which is constant for all observers: see the theory of relativity) is, to nine significant figures, exactly 299,792,458 metres per second (approximately one million km/h). As the speed of light is used to define the SI metre, this is the value by definition.
The speed of light is slower in any medium which is not a vacuum, and varies from medium to medium. This variation gives rise to (as a result of quantum mechanics, particularly the concept of a path of least action) the phenomenon of refraction.